Direct Nanoscopic Measurement of Laminar Slip Flow Penetration of Deformable Polymer Brush Surfaces: Synergistic Effect of Grafting Density and Solvent Quality

Abstract

A detailed quantitative nanoscopic description of soft surfaces under dynamic flow is lacking, despite its importance. To better understand the role of surface texture in nanoscopic mass transport in complex media, we used Forster resonance energy transfer in combination with total internal reflectance fluorescence microscopy (FRET-TIRFM) to directly measure laminar slip flow penetration depth (slip length) on poly(N-isopropylacrylamide) (pNIPAM) thin films (50−110 nm) of different grafting densities (0.60, 0.38, and 0.27 chain/nm 2 ) in solvents of different qualities created via cononsolvency in situ. Nontrivial synergistic interplay of grafting density and solvent quality on slip length was observed. Slip lengths are typically tens of nm (40−100 nm), increasing and then reaching a plateau with applied linear flow velocity (192−2,952 μm/s) regardless of experimental system. Slip length was systematically larger for lower density films, but the effect of grafting density was more significant in a good solvent than a poor solvent. Interestingly, however, the stagnant film thickness (polymer swollen thickness minus the slip length) collapsed to almost a singular value for a given grafting density regardless of solvent quality, likely suggesting a large gradient of segmental mobility at nonequilibrium. Moreover, we found that slip flow penetrates into soft pNIPAM surfaces more deeply in a good solvent than in a poor solvent and that this behavior was general and independent of grafting density. This behavior is counter to the notion that less interaction between a fluid (probe) and a solid surface promotes slip.

Luminescent Surface‐Tethered Polymer Brush Materials

“…Similar to post-polymerization modification, pre-polymerization modification involves the anchoring of functional moieties to the surface; however, in this case the functional moieties are tethered to a linker and the polymers are subsequently grown/tethered to the functional moiety itself. [73][74][75] This technique allows for the position of the functional moieties to be precisely known at all degrees of solvation.…”

“…As the acceptor solution penetrates the polymer brushes, FRET can be observed between the anchored donor and solvated acceptor and the degree to which the donor fluorescence is quenched can be used to determine slip length. This technique allowed them to investigate not only penetration depth (slip length) of the acceptor through the polymer brushes [73] but also the effect of polymer brush grafting density [75] and solvent quality [74,75] on the penetration depth.…”

Liquid–Solid Interfaces under Dynamic Shear Flow: Recent Insights into the Interfacial Slip

“…Usually, the interfacial slip can be greatly restrained by constructing the hydrogen-bonded network structure at the liquid−solid interfaces, which can explain why the dynamic shear flow has a weaker effect on the interfacially anchored hydrophilic polymer layers. 37,38,43,47,85 To strengthen the interfacial hydrogen bonding interaction, available approaches have been reported. A number of studies have indicated that the charged interaction can induce the formation of strong hydrogen-bonded network structures.…”

“…In general, the techniques can be divided into two main categories. One is to trace the flow near a boundary by tracer particles using confocal, total internal reflectance fluorescence microscopy (TIRFM) or stimulated emission depletion (STED). − The other is based on the force or displacement response, such as atomic force microscopy (AFM), surface forces apparatus (SFA), X-ray, and neutron reflectometry (NR). − Although the mutual interaction between tracer particle probes and solid surfaces might be an issue to be evaluated, such experimental results were of great value and worth discussing, from which the microscopic mechanism of the interfacial slip could be revealed.…”

“…As a ubiquitous molecular interaction, hydrogen bonding at the liquid–solid boundary layer also makes a sizable contribution to the interfacial energy and thus the interfacial slip, especially in aqueous solutions. Usually, the interfacial slip can be greatly restrained by constructing the hydrogen-bonded network structure at the liquid–solid interfaces, which can explain why the dynamic shear flow has a weaker effect on the interfacially anchored hydrophilic polymer layers. ,,,, To strengthen the interfacial hydrogen bonding interaction, available approaches have been reported. A number of studies have indicated that the charged interaction can induce the formation of strong hydrogen-bonded network structures. − In addition, increased numbers of hydrogen-bonding acceptor or donor groups (e.g., amide, carboxyl, hydroxyl, and others) at the interfaces can have a positive effect on the local interfacial hydrogen-bonded network structures.…”